C++ VC ATL STL Polymorphism Code: Generic ostream operator<<?

Victor Bazarov wrote:

On 6/19/2012 3:30 PM, Gerhard Fiedler wrote:

I want to be able to stream custom classes to ostreams like this:

MyClass foo;
myStream<< foo;

AIUI, this requires an operator<<() with this signature:

std::ostream&operator<<( std::ostream&str, MyClass const&printable );

In order to avoid creating such a function for every class, I tried a
template like this:

template< class Printable>
std::ostream&operator<<( std::ostream&stream,
    Printable const&printable )
{
    printable.print( stream );
    return stream;
}

(All the classes in question have a public member function "void print=

std::ostream&stream )".)

However, with this I get tons of errors like "'operator<<' is
ambiguous".

Is there a way to define this template function so that it is only
instantiated for classes that implement this print() function? (I coul=

make the name such that it is very unlikely that any other class has
such a name.)

Or is there another way to avoid having to define such a global
operator<< for every class with ostream support?

Have you considered using polymorphism at all, or is it out of fashion=

already?

Thank you for your response. In fact I didn't consider it at first,
reconsidered it after reading your post, and threw it out again :)

This is meant to work for an indeterminate number of classes of a bigger
system, some of which are not polymorphic at all for various reasons. I
don't want to put this constraint on all the classes that potentially
may want to use this interface.

If there was a way without polymorphism, I would prefer this.

Gerhard

Victor Bazarov wrote:

On 6/19/2012 3:30 PM, Gerhard Fiedler wrote:

I want to be able to stream custom classes to ostreams like this:

MyClass foo;
myStream<< foo;

AIUI, this requires an operator<<() with this signature:

std::ostream&operator<<( std::ostream&str, MyClass const&printable );

In order to avoid creating such a function for every class, I tried a
template like this:

template< class Printable>
std::ostream&operator<<( std::ostream&stream,
    Printable const&printable )
{
    printable.print( stream );
    return stream;
}

(All the classes in question have a public member function "void print=

std::ostream&stream )".)

However, with this I get tons of errors like "'operator<<' is
ambiguous".

Is there a way to define this template function so that it is only
instantiated for classes that implement this print() function? (I coul=

make the name such that it is very unlikely that any other class has
such a name.)

Or is there another way to avoid having to define such a global
operator<< for every class with ostream support?

Have you considered using polymorphism at all, or is it out of fashion=

already?

Thank you for your response. In fact I didn't consider it at first,
reconsidered it after reading your post, and threw it out again :)

This is meant to work for an indeterminate number of classes of a bigger
system, some of which are not polymorphic at all for various reasons. I
don't want to put this constraint on all the classes that potentially
may want to use this interface.

If there was a way without polymorphism, I would prefer this.

Gerhard

Re: Generic ostream operator<<?